This invention relates to the recovery and purification of metallic uranium scrap, and more particularly to a novel process and apparatus for the production of bulk metallic uranium from metallic uranium scrap, especially scrap of depleted uranium.
Natural uranium, i.e., the material that is mined from the earth, contains 0.005 percent uranium-234 (U.sup.234), 0.7 percent U.sup.235, and 99.295 percent U.sup.238. Depleted uranium "DU" is a byproduct of isotope separation processes, primarily the gaseous diffusion process, through which uranium containing a higher content of the U.sup.235 isotope, referred to as enriched uranium, is produced from natural uranium. Enriched uranium is used for reactor fuel and nuclear weapons. DU, the residue of the enrichment process, contains only a portion of the original U.sup.235 and U.sup.234 and is therefore "depleted" in these isotopes. The isotope distribution will vary with different enrichment goals. DU is a high density metal used as counterweights in some military and commercial aircraft, as components in some military projectiles, and as shielding material in containers in which highly radioactive material such as Co.sup.60 are stored and used. During production of the above items, DU scrap is produced.
The values of natural and enriched uranium metal range from $40 per pound to well over $10,000 per pound, depending upon purity and level of enrichment. Therefore, highly efficient recovery procedures, even though very expensive, are used for the recycling of these materials. On the other hand depleted uranium is worth $0.50 to $2.00 per pound in bulk, and no economically suitable recovery scheme has been developed for recovering and re-using this scrap. Instead, several thousand pounds per day of metallic scrap (turnings, crops, rejects, etc.) are placed in drums, packed with nonflammable material and transported to radioactive-waste dumps where the drums are buried. The burial of such radioactive waste is becoming very restricted and increasingly costly.
In "Reactor Handbook," Second Edition, Volume 1, pp 98-104, and Volume 2, pp. 379-391, the engineering aspects of industrial and governmental uranium processing is disclosed (i.e. UF.sub.4 reduction) in essentially the same manner in which uranium processing is currently being conducted, and in particular, identifying the importance of allowing time for phase separations of molten uranium from MgF.sub.2 or CaF.sub.2, either in derby or dingot reduction. This time for separation is achieved by preheating and by size and shape considerations only; no use is made of cleaning at temperature or of eutectic mixtures. CaF.sub.2, MgF.sub.2, and various oxides are identified as liners for graphite containers. Uranium chips along with trapped debris are pressed into briquettes prior to melting under vacuum along with other metallic uranium scrap. Oxide scums and volatile impurities are identified as sources of problems in the uranium remelting, and it is also recognized that graphite crucibles lead to carbon pickup. Oxide scums create problems also at the metal-salt interfaces during the original reductions of UF.sub.4 by calcium or magnesium.
In "Uranium Metallurgy" by W. D. Wilkinson, there is disclosed the melting of uranium chips in crucibles lined with magnesium zirconate covered with a protective layer of a mixture of calcium fluoride and magnesium fluoride; this melting yields ingots containing 400-600 ppm of carbon. This large carbon addition is grossly unacceptable for much uranium recycling; it is a consequence jointly of the use of an improper salt (MgF.sub.2) together with the use of an improper container material (graphite).